The present invention relates to a method and an apparatus for producing and delivering a gas mixture having a selected composition of a first gas and at least one second gas. In particular, the present invention is used for the dynamic production of gas mixtures.
Dynamic gas mixing is used for continuously filling cylinders with a gas mixture having a selected composition of a first (main) gas and at least one second gas. Usually the first gas, which has the highest concentration in the gas mixture, is provided in a main conduit as a main gas flow and the second gas is added into that main conduit, so that the first gas and the second gas are blended and form a gas mixture flow. The flow rate of the first gas and the flow rate of the second gas are set to such values that a gas mixture having approximately the desired composition is produced.
At selected time intervals or continuously the rate of flow of gas mixture and the composition of the gas mixture is measured as it passes a selected point. Thereby, the current concentration of each gas in the flowing gas mixture is determined. The flow rate measurement and gas mixture analysis results are used to determine the composition of the entire quantity of gas mixture that has passed the selected point. If the components in the accumulated quantity of gas mixture, that has passed the given point, are at the desired ratios, no adjustment of flow of any component of the gas mixture is necessary. If, however, the gas mixture has a composition that is beyond the predetermined composition limits, a signal is sent back to one or more flow control devices associated with gas lines that feed the first gas and the second gas into the main gas conduit to cause the flow control devices to adjust the rate of gas flow to cause the difference between the measured and targeted composition to be diminished. Analyses and flow rate adjustments are made frequently throughout the course of a filling activity, so that the composition of the gas mixture will be maintained within a narrow range.
The gas mixture is subsequently compressed and charged into a plurality of parallel aligned gas bottles. Furthermore, the apparatus may comprise purge valves and inert gas sources, so that the apparatus may be purged after each filling cycle.
Prior art document U.S. Pat. No. 5,836,632 discloses a method for dynamically filling gas cylinders with gas mixtures. The gases forming the gas mixture are separately introduced into a main conduit through individual delivering conduits at the same position of the main conduit. All gas components are added to the same space within the main conduit.
Prior art document U.S. Pat. No. 5,495,875 discloses a dynamic system for continuously filling a plurality of cylinders with a precise concentration of a vaporized liquid component blended into a gas or gas mixture. The different gas components may be added to the main gas flow in a main conduit in subsequent positions with respect to the flow direction in the main conduit. For mixing a plurality of second gases the main conduit has to have a certain length so that all gases may be added into the main conduit.
With the known systems it is not possible to produce a gas mixture which has a component with a concentration of a few percent and at the same time with a component which has a concentration of only a few ppm [parts per million] or even ppb [part per billion].
Therefore, equipment and methods are needed for producing gas mixtures containing second gases with a concentration from below a few hundreds ppm to a few percent.